Electric machine stator with liquid cooled teeth

ABSTRACT

A system for cooling the teeth of an electric machine stator. The stator includes a stator core that may be formed of a plurality of laminations. Each lamination has a plurality of back iron apertures, a plurality of tooth tip apertures, and a plurality of elongated apertures. When the laminations are assembled to form the stator core, the back iron apertures align to form back iron inlet channels and back iron outlet channels, and the tooth tip apertures align to form tooth tip cooling channels. The elongated apertures are L-shaped and connect the back iron inlet channels and back iron outlet channels to the tooth tip channels. Cooling fluid may flow, for example, axially through a back iron inlet channel, azimuthally and radially inward through an elongated aperture to a tooth tip, axially along a tooth tip channel, and to a back iron outlet channel through another elongated aperture.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 17/019,106, filed Sep. 11, 2020, entitled “ELECTRIC MACHINESTATOR WITH LIQUID COOLED TEETH”, which is a continuation of U.S. patentapplication Ser. No. 16/505,494, filed Jul. 8, 2019, now U.S. Pat. No.10,790,728, entitled “ELECTRIC MACHINE STATOR WITH LIQUID COOLED TEETH”,which is a continuation of U.S. patent application Ser. No. 15/010,879,filed Jan. 29, 2016, now U.S. Pat. No. 10,411,563, entitled “ELECTRICMACHINE STATOR WITH LIQUID COOLED TEETH”, which claims priority to andthe benefit of U.S. Provisional Application No. 62/110,314, filed Jan.30, 2015, entitled “ELECTRIC MACHINE STATOR WITH TRANSVERSE LIQUIDCOOLED TEETH”; the entire contents of each of the applicationsidentified in this paragraph are incorporated herein by reference.

FIELD

One or more aspects of embodiments according to the present inventionrelate to electric machines, and more particularly to a system forcooling an electric machine stator.

BACKGROUND

The continuous power to mass ratio (specific power) is an importantmetric for electric motors, especially for those used to power electricand hybrid vehicles. As this parameter is increased, motor mass can bereduced while maintaining a given level of performance. This providesboth direct and indirect economic benefits. Since power is equal totorque times speed (rpm), high specific power may be achieved by thecombination of high shaft speed and high torque per unit mass (highspecific torque). Since electrical and core magnetic frequencies may beproportionate to shaft speed and since magnetic losses may increaseapproximately with the square of these frequencies, core losses mayincrease rapidly with increasing speed. Likewise, since winding lossesmay be approximately proportionate to the square of the torque, thisloss component may increase rapidly with increasing torque. As a result,the operation of high specific power machines may be facilitated byefficient heat rejection for both the core and the winding.

Winding temperature may exceed the core temperature, and elevatedwinding temperatures may cause increased winding losses. Accordingly, ametric may be defined which involves winding hot-spot temperature andtotal loss within the stator. This metric, the stator thermalresistance, is defined as the temperature difference between the hottestpart of the stator winding and the cooling medium (e.g. inlet coolant)divided by the total stator heat dissipation. As the stator thermalresistance is lowered, the continuous power capability and hencecontinuous power rating of the overall machine may increase. As such,low stator thermal resistance may be helpful in achieving high specificpower.

In a related art liquid-cooled stator, the active core may be containedwithin a liquid cooled enclosure and the winding may be electricallyinsulated from the core via slot liners and electrical varnish. Heatproduced within the winding may be constrained to flow through a seriesof elements, such as electrical varnish and slot liners, each addingthermal resistance, before reaching coolant which flows within theenclosure. Both electrical varnish and slot liners may offer significantthermal resistances. Heat is received by the core teeth and flowsradially through the back iron and on to the enclosure.

For large diameter machines, both the tooth and back iron thermalresistances may be significant. The interface between the core and theenclosure may present yet another resistance element, as may thematerial of the enclosure itself. An additional resistance element isassociated with heat transfer from the interior surfaces of theenclosure to the coolant. The combination of such thermal resistanceelements may limit the performance of a motor. The temperature of thestator, and particularly the temperature of the stator teeth tips, mayalso affect the rotor, which may exchange heat with the stator byconduction, convection, and radiation heat transfer.

Thus, there is a need for an improved system for cooling a stator of anelectric motor.

SUMMARY

According to an embodiment of the present invention there is provided anelectric machine stator having an axis and including: a stator corehaving a plurality of layers, each of the layers having a back ironportion and a plurality of teeth, a tooth of the plurality of teeth of afirst layer of the plurality of layers having a first aperture forming afirst portion of a fluid channel.

In one embodiment, a layer adjacent the first layer has an apertureoverlapping the first aperture.

In one embodiment, the first portion of the fluid channel extendsradially within the tooth.

In one embodiment, the first portion of the fluid channel has adimension in an axial direction equal to a thickness of a layer of theplurality of layers.

In one embodiment, the first portion of the fluid channel has a firstsegment in a direction having a radial component with respect to theaxis and a second segment in a direction having an azimuthal componentwith respect to the axis.

In one embodiment, the stator core has a total volume, and the statorcore has a plurality of fluid channels, including the fluid channel,having a total fluid contact area, and the total volume divided by thetotal fluid contact area is less than one inch.

In one embodiment, the first layer has a second aperture having amirror-image shape of the first aperture.

In one embodiment, a second layer of the plurality of layers has thesame shape as the first layer of the plurality of layers.

In one embodiment, the back iron portion of each of the layers has aplurality of second apertures, and the second apertures overlap onadjacent layers to form a plurality of substantially axial fluidpassages.

In one embodiment, the first aperture overlaps one of the secondapertures.

In one embodiment, the stator includes a flow director configured todirect fluid flow into, or receive fluid flow from, a subset of theplurality of substantially axial fluid passages.

In one embodiment, the flow director is a layer at one end of the statorcore.

In one embodiment, the fluid channel includes: a first axial segmentthrough the back iron portion of a first subset of the plurality oflayers; a first azimuthal segment in the back iron portion of the firstlayer; a first radial segment being the first portion of the fluidchannel; a second axial segment extending through a respective tooth ofeach of a second subset of the plurality of layers; a second radialsegment within a tooth of a second layer of the plurality of layers; asecond azimuthal segment in the back iron portion of the second layer;and a third axial segment through the back iron portion of a thirdsubset of the plurality of layers.

In one embodiment, the plurality of layers is a plurality oflaminations.

In one embodiment, the plurality of layers is a plurality of turns of anedge-wound strip.

In one embodiment, the plurality of layers is a plurality of turns of aface-wound strip.

In one embodiment, the teeth of the plurality of teeth are narrower at afirst end of the strip than at a second end of the strip, and wherein awidth of a slot between adjacent teeth at the first end of the strip isthe same as a width of a slot between adjacent teeth at the second endof the strip.

In one embodiment, each tooth of the plurality of teeth of each of theplurality of layers extends radially inward from the back iron portion.

In one embodiment, each tooth of the plurality of teeth of each of theplurality of layers extends radially outward from the back iron portion.

In one embodiment, the electric machine includes: a stator winding; andan electrically insulating resin having a thermal conductivity greaterthan about 0.4 W/m/° C., wherein the resin fills, with a void fractionless than about 10%, a space between the stator core and the statorwinding, and/or a gap between a pair of adjacent layers of the pluralityof layers.

In one embodiment, the electric machine includes a sealing compound in agap between two adjacent layers of the plurality of layers.

In one embodiment, each of the plurality of layers has an aperture, of aplurality of apertures, in a tip of a respective tooth, wherein theplurality of apertures includes the first aperture, and wherein theapertures of the plurality of apertures overlap to form a second portionof a fluid channel, the second portion of the fluid channel comprisingthe first portion of the fluid channel, and the second portion of thefluid channel being substantially axial

In one embodiment, all of the layers of the plurality of layers areidentical, and each layer of the plurality of layers is clocked by onetooth pitch relative to an adjacent layer.

According to an embodiment of the present invention there is provided anelectric machine including: a rotor having an axis of rotation; and astator having an axis, the axis of the stator being the axis of rotationof the rotor, the stator having: a stator core having a plurality oflayers, each of the layers having a back iron portion and a plurality ofteeth, a tooth of the plurality of teeth of a first layer of theplurality of layers having a first aperture forming a first portion of afluid channel.

According to an embodiment of the present invention there is provided anelectric machine including: a rotor; a stator having a stator corehaving a plurality of teeth; channel means for channeling a fluidthrough the teeth of the stator core; and pumping means for supplyingthe fluid to the channel means.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated and understood with reference to the specification, claims,and appended drawings wherein:

FIG. 1A is a cross-sectional view of an electric machine, according toan embodiment of the present invention;

FIG. 1B is another cross-sectional view of an electric machine,according to an embodiment of the present invention;

FIG. 2A is a plan view of a stator lamination, according to anembodiment of the present invention;

FIG. 2B is a plan view of another stator lamination, according to anembodiment of the present invention;

FIG. 3 is a perspective exploded view of a stack of stator laminations,according to an embodiment of the present invention;

FIG. 4A is a perspective exploded view of a portion of a stack of statorlaminations, according to an embodiment of the present invention;

FIG. 4B is a perspective view of a portion of a flow pattern within astack of laminations, according to an embodiment of the presentinvention;

FIG. 5A is a plan view of a portion of a strip before winding, accordingto an embodiment of the present invention;

FIG. 5B is a plan view of three portions of a strip before winding,according to an embodiment of the present invention;

FIG. 6 is an exploded view of a stator core formed from an edge-woundstrip, according to an embodiment of the present invention;

FIG. 7 is an exploded view of a stator core formed from a face-woundstrip, according to an embodiment of the present invention;

FIG. 8 is an exploded view of a stator core formed from a face-woundstrip, along with a manifold structure, according to an embodiment ofthe present invention;

FIG. 9 is an exploded view of a stator core formed from a face-woundstrip, along with a manifold structure and end-turn cooling elements,according to an embodiment of the present invention

FIGS. 2A-2B and 5A-5B are each drawn to scale for a respectiveembodiment.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of anelectric machine stator with transverse liquid cooled teeth provided inaccordance with the present invention and is not intended to representthe only forms in which the present invention may be constructed orutilized. The description sets forth the features of the presentinvention in connection with the illustrated embodiments. It is to beunderstood, however, that the same or equivalent functions andstructures may be accomplished by different embodiments that are alsointended to be encompassed within the spirit and scope of the invention.As denoted elsewhere herein, like element numbers are intended toindicate like elements or features.

Some embodiments of the present invention eliminate or reduce thermalresistances such that the overall winding to coolant thermal resistanceis significantly reduced—thus enabling a significant increase incontinuous specific power. More specifically, in some embodimentscoolant is introduced within the stator core teeth such that heattransfer path lengths are held to very small values and such thatcoolant head losses are also maintained to relatively low values.

Referring to FIG. 1A, in one embodiment the stator consists of magneticstator core 102, winding 104, inlet manifold 106, and outlet manifold108. In turn, stator core 102 consists of stacked interior laminations110 and end laminations 112. Back iron apertures 130 (in the back ironof the stator core) overlap to form back iron channels 132. The crosssection of FIG. 1A is taken through a winding slot in the upper portionof the stator core 102, and through an opposite winding slot in thelower portion of the stator core 102.

FIG. 1B is a cross section taken through the center of a tooth 120 inthe upper portion of the stator core 102, and through the center of anopposite tooth in the lower portion of the stator core 102. In FIG. 1B,a plurality of elongated apertures 122, each of which extends radiallyinto a stator tooth at the location of one lamination, are visible incross section. Moreover, each tooth which does not include a radialportion of one of the elongated apertures 122 contains a tooth tipaperture 126; the tooth tip apertures 126 align, when the interiorlaminations 110 are stacked, to form axial tooth tip channels 128. Asused herein, the “axis” of a stator for a rotary motor is the axis ofrotation that a rotor used with such a stator will have, and the “axial”direction is parallel to this axis. The embodiment of FIGS. 1A and 1Bshows a motor having a rotor inside the stator; in other embodiments thestator may instead be inside the rotor.

The channels in the stator may act as fluid channels; cooling fluid maycirculate through the channels to cool the stator. Cooling fluid, or“coolant”, may be supplied, (e.g., from a cooling fluid pump) to themotor at a cooling inlet 144 and may return (e.g., to a cooling fluidreservoir, via a heat exchanger) through a cooling outlet 146. The samecooling fluid may also cool the rotor via a cooling circuit that may bein parallel with the stator cooling circuit as shown, and that may beconnected to cooling channels in the rotor through rotary fluidcouplings connected to the shaft of the rotor as shown. The motor may besealed, e.g., by two end bells 148 and a sealing sleeve 150 each ofwhich may seal against the inlet manifold 106 and/or the outlet manifold108.

In some applications, functional elements of the motor may be part of anassociated element and vice versa. For example, if the motor is coupledto and drives a gear box, a gear pinion may be an integral part of therotor shaft, while the corresponding bearing is part of the gear box. Ina like manner, it is possible that one or both of the fluid couplings orone or both stator manifolds are parts of external elements, such asgear boxes, inverters, or tandem machines. In one embodiment theelectric machine is a permanent magnet machine with a permanent magnetrotor, and the stator includes some or all of the features describedherein. In one embodiment a gearbox that is part of or coupled to theelectric machine includes, or supports, an inlet manifold 106 or outletmanifold 108, an end bell, a bearing, and/or a fluid coupling.

Each manifold 106, 108 includes a cavity 134 which communicates with arespective subset of the back iron channels 132. The cavity 134 maycontain heat transfer elements 136 which enhance heat transfer betweenthe manifold material and the coolant. These elements may be, orinclude, for example, fins, ribs, or stacked laminations which includefluid channels. Each manifold 106, 108 is sealed to a respective face ofthe stator core 102 using a gasket, O-rings or a sealant. In oneembodiment, the two manifolds are drawn together by two or more tie rods(not shown). The manifolds may also provide cooling for the winding endturns. Significantly better cooling may be possible where the manifoldcavity contains a multi-layered cooling element which has a large areain contact with the cooling fluid.

FIG. 2A shows the design of the interior laminations 110 in oneembodiment. Each interior lamination 110 contains lamination slots 114which align, when the interior laminations 110 are stacked, to formwinding channels which in turn receive winding 104. Likewise, laminationteeth 118 align, when the interior laminations 110 are stacked, to formteeth 120. Every n^(th) lamination tooth 118 (where n=4 in theembodiment of FIG. 2A) includes an elongated aperture 122 which extendsinto a back iron portion 124 of the interior lamination 110. Back ironapertures 130, located within the back iron portion 124 along slotcenterlines, align when stacked to form axial back iron channels 132.Half of these back iron channels 132, consisting of every other backiron channel 132 (and referred to herein as the “odd-numbered” back ironchannels), may operate as inlet channels and be directly connected tothe inlet manifold 106; the remaining back iron channels 132 may operateas outlet channels and be directly connected to the outlet manifold 108.In any of the interior laminations 110, half of the elongated apertures122, consisting of every other elongated aperture 122 (and referred toherein as the “even-numbered” elongated tooth apertures) are shaped andpositioned such that they form contiguous paths with even numbered backiron channels 132 (the back iron outlet channels) when interiorlaminations 110 are stacked. Likewise, odd numbered elongated apertures122 are shaped and positioned such that they form contiguous paths withodd numbered back iron channels 132 (the back iron inlet channels) wheninterior laminations 110 are stacked.

In the embodiment of FIG. 2A, the number of back iron apertures 130 plusthe number of elongated apertures is equal to the number of teeth, andall of the teeth are cooled. In other embodiments, fewer than all of theteeth may be cooled, e.g., every other tooth, or every third tooth maybe cooled. In such an embodiment, the number of back iron apertures 130plus the number of elongated apertures may be an integer fraction of thenumber of teeth, e.g, there may be ½, ⅓, or ¼ as many apertures that areeither back iron apertures or elongated apertures as there are teeth.

Referring to FIG. 2B, each end lamination 112 may also include a backiron portion 124 and a plurality of teeth 118, as well as a plurality ofback iron apertures 130, of which there may be half as many as the totalof the number of back iron apertures and the number of elongatedapertures in an interior lamination 110. The end lamination 112 at theinlet end of the stator (referred to herein as the inlet end lamination112) may be aligned so that the back iron apertures 130 of the inlet endlamination 112 are aligned with the back iron inlet channels. Similarlythe end lamination 112 at the outlet end of the stator (referred toherein as the outlet end lamination 112) may be aligned so that the backiron apertures 130 of the outlet end lamination 112 are aligned with theback iron outlet channels. Each of the interior laminations 110 and eachof the end laminations 112 may include a witness mark 127 on the outeredge of the lamination.

In one embodiment each back iron aperture 130 (FIGS. 2A-2B) is a0.10″×0.10″ square, and each witness mark 127 (FIGS. 2A-2B) is a 0.050″diameter semi-circle.

FIG. 3 shows the stacking of a plurality of interior laminations 110between two end laminations 112, in one embodiment. Both odd and evennumbered elongated apertures 122 are contiguous with tooth tip channels128 (FIG. 4B) that are formed by overlapping tooth tip apertures 126. Inone embodiment, all interior laminations 110 are identical. For thiscase, as shown in FIG. 3, stacking may be such that the j+1^(th)lamination is rotated one tooth pitch clockwise (or counter-clockwise)relative to the j^(th) lamination. End laminations 112 serve as inletand outlet flow directors. As used herein, a “flow director” is astructure that allows fluid to flow into, or out of, some, but not all,of the axial back iron channels in a structure having such channels orthe radial back iron channels in a structure (e.g., in an axial gapstator core, discussed in further detail below) having such channels. Inthe embodiment of FIG. 3, for example, the inlet end lamination allowsfluid to flow from the inlet manifold 106 only into the back iron inletchannels, and the outlet end lamination allows fluid to flow into theoutlet manifold 108 only from back iron outlet channels. In someembodiments all of the laminations of a stator are the same, and havethe configuration of an interior lamination as illustrated in FIG. 2A.In these embodiments a flow director may be, for example, a manifoldhaving internal protrusions in the manifold channel that extend againstthe surface of (or into the apertures of) an end lamination at some ofthe apertures (e.g., at all of the tooth tip apertures 126, at all ofthe elongated apertures 122, and at a subset of the back iron apertures130) blocking some of the apertures, so that fluid is able to flow intoor out of only a set of back iron apertures including half as manyapertures as the total number of back iron apertures 130 and elongatedapertures 122 of the lamination.

In particular, at the inlet end of stator core 102, a first endlamination 112 (the inlet end lamination) allows cooling fluid to enterodd numbered back iron channels 132 (the back iron inlet channels),while blocking flow to or from the even numbered channels. Likewise atthe outlet end of stator core 102, a second end lamination 112 (theoutlet end lamination) allows coolant to exit from even numbered backiron channels 132 (the back iron outlet channels), while blocking flowto or from the odd numbered channels. In one embodiment, both of theseend laminations, the first end lamination 112, and the second endlamination 112, are identical. The second end lamination 112 is rotatedone tooth pitch clockwise (or counterclockwise) relative to the firstend lamination 112. Inlet manifold 106 serves to distribute receivedcoolant flow to odd numbered back iron channels 132. Likewise, outletmanifold 108 collects coolant received from even numbered back ironchannels 132. In FIG. 3, each of the interior laminations 110 is shownas being rotated by one tooth counterclockwise relative to the precedinglamination, along the length of the stator. In other embodiments thechange in orientation of successive laminations may be greater than onetooth, and/or may be clockwise instead of counterclockwise. In someembodiments, some of the interior laminations may not be rotatedrelative to the adjacent interior laminations. For example, groups of klaminations (where k is a positive integer greater than 1) may bealigned with each other (i.e., not rotated relative to each other), andadjacent groups of k may be rotated relative to each other by one ormore teeth. If k is 2, for example, then pairs of interior laminationsare aligned, so that each elongated aperture 122 is aligned with acorresponding elongated aperture 122 of the other lamination of thepair, and the two elongated apertures form a radial tooth coolingchannel that has an axial width twice as great as the thickness of alamination.

In some embodiments all of the interior laminations 110 are identical towithin manufacturing tolerances. In these embodiments, the witnessmarks, of the interior laminations 110, on the outer surface of theassembled stator core form a spiral, the witness mark on each laminationbeing advanced by, e.g., one tooth pitch clockwise or counterclockwiserelative to the preceding lamination along the length of the statorcore. This may make it possible to verify that the laminations have beenassembled correctly by a visual inspection, as, for example, any singlelamination that is clocked by the wrong amount relative to its neighborswill have a witness mark that is offset from the spiral formed by theremaining laminations, if they are properly aligned. In otherembodiments the interior laminations 110 may have alignment notches inaddition to the witness marks, and the alignment notches may be, on eachsuccessive lamination, positioned at a different point on thecircumference relative to the pattern of elongated apertures 122. Theend laminations 112 may also have alignment notches. The alignmentnotches on the interior laminations 110 and on the end laminations 112may be placed such that when the interior laminations 110 and the endlaminations 112 are correctly assembled as part of the stator core, allof the alignment notches are aligned axially. In this embodiment theinterior laminations 110 and the end laminations 112 may be installed ina housing or assembly fixture with a corresponding interior axial ridgeto effect and maintain the correct azimuthal alignment while thelaminations are bonded together and/or while the stator winding 104 isput in place.

The embodiment of FIG. 3 establishes a large number of parallel coolantpaths, each of which starts with coolant received from an axial inletchannel, then through a radially directed tooth channel, followed by ashort axial path through the tooth tip channel, and then through asecond radial tooth channel to an adjacent back iron outlet channel. Inone embodiment, to achieve low thermal resistance between the windingand the core, conventional slot liners may be replaced by a thermallyconductive powder coat applied to the core slots, and a thermallyconductive potting resin, such as a thermally conductive epoxy may bemolded under pressure within all parts of the winding, including the endturn. This combination may provide tight thermal coupling between allparts of the winding and the core; both the active winding elements aswell as the end turn may be cooled such that very high current densitiesmay be maintained without any portion of the winding exceeding criticaltemperature limits. In some embodiments, the interior laminations 110and the end laminations 112 are bonded together and sealed with abonding agent or sealing compound that is applied to the surfaces of thelaminations prior to assembly or that may be applied to the stator coreafter it is assembled, and seep into the inter-lamination gaps and set,preventing cooling fluid from leaking out of the stator core through theinter-lamination gaps. In some embodiments the powder coating or thepotting resin mentioned above may seal the stator core, in addition to,or instead of, the bonding agent or sealing compound. A scavenge pump152 (FIGS. 1A and 1B) may be used to recirculate any cooling fluid thatescapes from the stator core or from the rotor core. In some embodimentsthe cooling fluid is a low viscosity oil such as automatic transmissionfluid (ATF) or transformer oil. In some embodiments, low thermalimpedances may be achieved in this manner between the coolant and thecore; furthermore, since cooling of the core teeth is provided, thethermal resistance of the teeth may be virtually eliminated from theoverall thermal circuit.

In a three phase machine, the number of teeth and slots may be amultiple of six. Small machines may have as few as six teeth, whilelarge machines may have 60 or more. As the number of teeth is increased,current harmonics may be reduced while heat transfer between the windingand the core may be improved due to the increased interface area andreduced heat flow length. In one embodiment there are N_(t) teeth (N_(t)being a positive integer), and the total number of apertures in, orextending into, the back iron (i.e., the total number of back ironapertures 130 and elongated apertures 122) of each of the interiorlaminations 110 is also N_(t); half of these serve to form back ironinlet channels and the other half serve to form back iron outletchannels. In each of the interior laminations 110, every n^(th) toothcontains an elongated aperture 122 which serves as a coolant channel.Here, n is a positive integer between 2 and N_(t)/2 that divides evenlyinto N_(t). In each of the interior laminations 110, half of theelongated apertures 122 receive coolant from odd numbered back ironchannels 132 and carry coolant radially inward (for a radial-gap statorwith inward-facing teeth) to the tooth tip channel 128 (FIG. 4B).Coolant then flows axially through this tooth tip channel for a distanceequal to n lamination thicknesses and then flows radially outwardthrough an even numbered elongated aperture 122 and on to an evennumbered back iron channel 132 and finally flows axially on to theoutlet manifold.

In an embodiment with N_(t) teeth and N_(L) total interior laminations110, the approximate number of radial tooth cooling channels (eachformed by an elongated aperture 122) is N_(L)*N_(t)/n. The total wallarea associated with these channels may be significant. For example, inone embodiment in which each elongated aperture has an associated wallarea of 0.5 in², N_(L)=600, N_(t)=48, and n=6, the total fluid contactsurface is approximately 0.5*600*48/6=2400 in² or nearly 17 ft². Theratio of the volume of the stator core to total fluid contact surfacemay be less than 1 inch. The section associated with coolant flow isalso relatively large, thus enabling relatively low ratios of head lossto flow rate. Heat flow distances may be short—as demonstrated by theabove example, in which the maximum heat flow length within the toothelements may be about 0.10″. Tooth tip apertures may be relativelynarrow so that magnetic sections are minimally reduced. The crosssection of the tooth tip aperture may be made to be about twice thesection presented by the radial tooth cooling channel, and, as a result,the head loss due to the tooth tip channel may be relatively small. As nis reduced to provide more elongated apertures, heat transfer may beimproved, while magnetic sections may be reduced. This may make itpossible to select n such that a desired trade-off criterion is met.

In some embodiments, an analogous set of interior laminations and endlaminations are stacked to form a stator core for an “inside-out” motorin which teeth and slots face radially outward. In other embodiments astator core for a linear machine is formed by stacking suitableanalogous laminations.

A stator core formed by stacking the laminations of FIGS. 2A and 2B maycontain a number of parallel fluid paths between the inlet manifold andthe outlet manifold, one of which is illustrated in FIG. 4A. The fluidpath illustrated includes a first axial portion 410 passing through aback iron aperture 130 in the first end lamination 112, followed by afirst L-shaped portion 415 following a first elongated aperture 122 inan interior lamination 110, followed by a second axial portion 420through a plurality of tooth tip apertures 126, followed by a secondL-shaped portion 425, in a second elongated aperture 122, followed by athird axial portion 430 through a plurality of overlapping back ironapertures 130 forming a back iron outlet channel. In FIG. 4B, aplurality of such fluid paths are shown in a perspective view showingtwo back iron channels 132 including a back iron inlet channel and aback iron outlet channel, a tooth tip channel 128 and six L-shapedchannels formed by respective elongated apertures 122. Fluid flows inthrough the back iron inlet channel and then through any of threeparallel paths through respective elongated apertures 122 to the toothtip channel 128. Within the tooth tip channel 128 fluid flows to thenearest elongated aperture 122 connected to the back iron outletchannel, through the channel formed by that elongated aperture 122 tothe back iron outlet channel, and out through the back iron outletchannel. FIG. 4B shows only a relatively small number of channels andfluid paths for clarity; as explained above, in some embodiments astator core may include a significantly greater number of channels andfluid paths.

In some embodiments, instead of being formed as a stack of laminations,the stator core is formed as an edge-wound strip. As used herein, and“edge-wound” strip has the shape of a SLINKY™, or, on one turn, of apiston ring, the edge-wound strip being a strip with a length, a width,and a thickness, the length being greater than the width, and the widthbeing greater than the thickness, the strip being wound into a helicalshape, with the curvature of the strip at every point being parallel tothe width direction. Referring to FIG. 5A, the strip prior to windingmay have the shape shown along a portion of its length, with teeth thatoverlap, when the strip is wound, to form the stator core teeth. FIG. 6is an exploded view of a stator core formed from an edge-wound strip,with successive turns (or “layers”) pulled apart axially to makeapertures of the inner turns visible. As used herein, a “layer” referseither to a turn of a wound strip or to a lamination of a laminatedstructure. The operation of the wound strip assembly of FIG. 6 isanalogous to that of the assembly of stacked laminations illustrated,for example, in FIGS. 3 and 4A. The strip has a portion (not shown inFIG. 5A) at each of the two ends of the strip (e.g., a portion 48 teethlong, in an embodiment analogous to that of FIGS. 2A-3) that forms thefirst and last turns 612 of the wound strip and acts as a flow directorinto or out of either the even-numbered or odd numbered back ironchannels 132, each of which is formed by overlapping back iron apertures130 in the strip. Each of these two portions of the strip may lack toothtip apertures 126 and elongated apertures 122, and may have back ironapertures 130 spaced twice as far apart as on the remainder of the strip(i.e., back iron apertures 130 separated by twice the tooth pitch).

The remainder of the strip may have tooth apertures and may form theinterior turns 610 of the wound strip. The back iron portion of thestrip may have notches or slots (e.g., slots on the bottom edge of thestrip as shown in FIG. 5A, which may become the outer edge of the stripwhen wound) to facilitate winding (i.e., to reduce the degree to whichthe strip must stretch or be compressed when wound). In high pole-countmotor, the back iron may be sufficiently narrow that bending may bereadily possible without such notches, and the strip may be fabricatedwithout them (as illustrated, for example, in FIG. 5A). In anotherembodiment, the strip of FIG. 5A may be formed into an edge-wound stripin which the teeth point radially outwards; such a strip may be used asa stator core for an “inside out” radial gap motor in which the statoris inside the rotor. In one embodiment, the notches and apertures of thestrip of FIG. 5A are formed while it is being wound (e.g., formed by asuitable punch or set of punches adjacent to a machine used to wind thestrip), so that the punching of the features of the strip may besynchronized with the winding of the turns, to preserve alignmentbetween the features of successive turns.

In another embodiment, a strip illustrated in FIG. 5B may be formed intoa face-wound structure illustrated in FIG. 7, which may for example bethe stator core of an axial-gap electric machine. Some broken lines areomitted from FIG. 7 for clarity. As used herein, a “face-wound” strip isa structure that has the shape of electrician's tape, being a strip witha length, a width, and a thickness, the length being greater than thewidth, and the width being greater than the thickness, the strip beingwound into a spiral shape, with the curvature of the strip at everypoint being parallel to the thickness direction. In this embodiment, thepitch of the teeth may increase along the length of the strip (as shownin FIG. 5B), so that as it is wound, the number of teeth on each turnremains constant as the diameter of the partially wound strip increases.The fluid flow in this structure may be analogous to the flowillustrated in FIGS. 4A and 4B, with an exemplary fluid path beginningin an inlet manifold extending around the outside of the structure andfeeding radial back iron inlet channels, that include every other one ofa set of radial back iron channels. From each back iron inlet channeleach of one or more elongated apertures 122 allows fluid to flowazimuthally away from the back iron inlet channel and axially to arespective tooth tip. The fluid then flows radially inward through oneor more tooth tip apertures 126, and axially back to the back ironthrough another elongated aperture 122, connected to an adjacent, radialback iron outlet channel, to a manifold inside the inner diameter of thestator core. In some embodiments the width of the winding slots isconstant along the length of the strip, whereas the width of the teeth(and the tooth pitch, as mentioned above) increases. In some embodimentsthe dimensions of the apertures are constant along the length of thestrip; in other embodiments one or more dimensions of one or more of (i)the back iron apertures, (ii) the tooth tip apertures, and (iii) theelongated apertures, vary along the length of the strip (e.g., theapertures may become wider as the teeth become wider).

Referring to FIG. 8, in one embodiment, fluid is supplied to andreturned from an axial-gap stator core using a manifold structure 816that has an inlet port 824 and an outlet port 826 both on the outersurface of the manifold structure (instead of having one of the ports onthe inner surface of the manifold structure). The manifold structure 816may include a first semi-circular outer fluid channel 818 and a secondsemi-circular outer fluid channel 820, separated by two partitions 822.The first outer fluid channel 818 is fed by the inlet port 824 and actsas the fluid channel of an inlet manifold, and the second outer fluidchannel 820 is evacuated by the outlet port 826 and acts as the fluidchannel of an outlet manifold. The stator core 802 then operates as twosemi-annular halves, a first semi-annular half connected to the inletport 824 and a second semi-annular half connected to the outlet port826. Coolant flow is generally radially inward within the firstsemi-annular half, so that it flows from the first semi-circular outerfluid channel 818 inwards through the first semi-annular half of thestator core, and into an inner fluid channel 832 (formed at the innerdiameter of the manifold structure 816). In the inner fluid channel 832fluid flows azimuthally from the first semi-annular half to the secondsemi-annular half. The coolant then flows generally radially outwardwithin the second semi-annular half, flowing from the inner fluidchannel 832 outward through the second semi-annular half of the statorcore to the second outer fluid channel 820, and then to the outlet port826. One exemplary fluid path in the first semi-annular half of thestator core may include a radial portion along which the fluid flowsthrough the inlet flow director (the outermost turn of the stator core)and through the back iron apertures 130 of several turns of the woundstrip (back iron apertures 130 that overlap to form a back iron inletchannel), then through a first elongated aperture 122 axially to the endof a stator tooth, then radially inward (or outward—see FIG. 4B) throughone or more stator tooth tip apertures 126, then through a secondelongated aperture 122 to the back iron and into a back iron outletchannel (formed by overlapping back iron apertures 130 in several turnsof the wound strip), and then radially inward through the back ironoutlet channel, to the inner fluid channel 832. In another embodimentthe manifold structure may lack the partitions 822 and (instead ofhaving an inlet port 824 and an outlet port 826 both on the outersurface of the manifold structure) have, e.g., an inlet on the outersurface of the manifold structure, and an outlet on the inner surface ofthe manifold structure.

Referring to FIG. 9, in one embodiment, wound strip end turn coolingstructures 912, 914 are used to cool the stator end turns in an axialgap electric machine. An outer cooling structure 912 has a plurality ofnarrow apertures 915 in every other turn of the wound strip. Successiveturns of the outer cooling structure 912 are shown pulled apart radiallyto make apertures of the inner turns visible. These align to form a setof substantially radial inlet channels alternating with a set ofsubstantially radial outlet channels. Each of the outermost turn 928 andthe innermost turn of the outer cooling structure 912 has half as manynarrow apertures as the intervening turns with narrow apertures. Theapertures of the outermost turn of the outer cooling structure 912 arealigned with the inlet channels and the apertures of the innermost turnof the outer cooling structure 912 are aligned with the outlet channels.Alternating with the intervening turns having narrow apertures 915 areturns having wide apertures 916, each wide aperture bridging two narrowapertures in one or two adjacent turns. Fluid flows through theoutermost turn into the inlet channels, then, from each inlet channel,in parallel fluid paths through a plurality of azimuthal channels formedby the long apertures, to the outlet channels, and then radially inwardin the outlet channels. The azimuthal channels may provide significantsurface area at which heat may be transferred from the surface of thewound strip to the coolant.

The stator 902 of an axial gap electric machine may have a stator core904 formed of a face-wound magnetic strip, with slots 906 in one facefor the stator winding 908. The back iron 910 of the stator 902 mayhave, as in the embodiment of FIG. 8, back iron apertures 130, and thestator core may also have, on the interior turns or laminations, toothtip apertures and elongated apertures that overlap with each other toprovide fluid paths for stator tooth cooling. The end turns of thestator winding 908 may be encapsulated in a thermally conductive pottingresin 909 that may provide a heat flow path from the end turns to theend turn cooling structures 912, 914. In one embodiment the same pottingresin is also molded under pressure into the winding slots to fill gapsthat otherwise may exist between the conductors of the winding and thelayers of the stator core. The thermally conductive potting resin 909may also seal the stator core 904, to prevent coolant from leaking outof the fluid channels in the stator core 904.

From the outer cooling structure 912 coolant may flow into the statorcore, either as a result of the apertures of the innermost turn of theouter cooling structure 912 being aligned with corresponding backchannel apertures of the outermost turn of the stator core, or as aresult of a gap between the innermost turn of the outer coolingstructure 912 and the outermost turn of the stator core. After passingthrough the stator core, the coolant flows through an inner coolingstructure 914 which has a structure analogous to that of the outercooling structure 912. As in the embodiment of FIG. 8, coolant may besupplied to, and may return from, the combination of the outer coolingstructure 912, the stator core 802 and the inner cooling structure 914,by a manifold structure 816 that has an inlet port 824 and an outletport 826 both on the outer surface of the manifold structure 816. Inother embodiments the face-wound structures of FIG. 9 may be replacedwith analogous structures formed of cylindrical laminations.

The heat transfer elements 136 of FIG. 1 may be edge-wound or laminatedstructures analogous to the outer cooling structure 912. For example, aheat transfer element 136 may include first and last laminations thatact as inlet and outlet flow directors, and a set of alternatinginterior laminations of which every other one has narrow apertures andthe remaining ones have wide apertures. The narrow apertures may overlapto form alternating inlet and outlet channels, the inlet channels beingfed by the inlet flow director (which has half as many apertures as aninterior lamination, the inlet flow director apertures being alignedwith the inlet channels) and the outlet flow director providing ananalogous flow path out of the structure from the outlet channels. Eachinlet channel may then be connected to each adjacent outlet channel by aplurality of parallel cooling passages formed by the wide apertures.

In some embodiments, the fluid may flow along paths different from thoseshown in FIGS. 4A and 4B. For example, fluid may flow axially in eachtooth, in a respective tooth tip channel along the entire length of thestator; the channels may be connected to suitable inlet and outletmanifolds at the ends. In another embodiment, the elongated aperturesand the corresponding fluid flow may differ from that of FIGS. 4A and 4Bin that, after the fluid flows axially along one tooth, it flowsradially outward to the back iron, azimuthally to a second tooth,radially inward to the tooth tip, and axially along the tooth tip of thesecond tooth before returning, via an elongated aperture, to a back ironoutlet channel. In another embodiment, fluid may flow, within onelamination, from a back iron channel 132 radially inward to a tooth tipand back out to the back iron, following, for example, a U-shapedaperture within the tooth. In some embodiments the apertures have shapesthat differ from those illustrated; the elongated apertures may becurved instead of angular, for example, and the back iron apertures maybe round or rectangular instead of square.

Although exemplary embodiments of an electric machine stator withtransverse liquid cooled teeth have been specifically described andillustrated herein, many modifications and variations will be apparentto those skilled in the art. Accordingly, it is to be understood that anelectric machine stator with transverse liquid cooled teeth constructedaccording to principles of this invention may be embodied other than asspecifically described herein. The invention is also defined in thefollowing claims, and equivalents thereof

What is claimed is:
 1. An electric machine stator having an axis andcomprising: a stator core having a plurality of layers, each of thelayers having a back iron portion and a plurality of teeth, a tooth ofthe plurality of teeth of a first layer of the plurality of layershaving a first aperture forming a first portion of a fluid channel,wherein the fluid channel includes: a first segment, the first segmentbeing substantially parallel to the first layer, and the first segmentcomprising the first portion of the fluid channel; a second segment, thesecond segment being substantially parallel to a second layer of theplurality of layers; and a third segment, the third segment extendingthrough a tooth of a third layer of the plurality of layers, wherein:the third layer is between the first layer and the second layer, thethird segment connects the second segment and the first segment, afourth layer of the plurality of layers is at one end of the stator coreand has no aperture, in any tip of any tooth, that is in fluidcommunication with the fluid channel, the stator is configured tointeract with a rotor through an air gap, and the fluid channel is notin fluid communication with the air gap.
 2. The electric machine statorof claim 1, wherein a layer adjacent the first layer has an apertureoverlapping the first aperture.
 3. The electric machine stator of claim1, wherein the first portion of the fluid channel extends radiallywithin the tooth.
 4. The electric machine stator of claim 3, wherein thefirst portion of the fluid channel has a dimension in an axial directionequal to a thickness of a layer of the plurality of layers.
 5. Theelectric machine stator of claim 1, wherein the first portion of thefluid channel has a first segment in a direction having a radialcomponent with respect to the axis and a second segment in a directionhaving an azimuthal component with respect to the axis.
 6. The electricmachine stator of claim 1, wherein the stator core has a total volume,and the stator core has a plurality of fluid channels, including thefluid channel, having a total fluid contact area, wherein the totalvolume divided by the total fluid contact area is less than one inch. 7.The electric machine stator of claim 1, wherein the first layer has asecond aperture having a mirror-image shape of the first aperture. 8.The electric machine stator of claim 1, wherein a second layer of theplurality of layers has the same shape as the first layer of theplurality of layers.
 9. The electric machine stator of claim 1, whereinthe back iron portion of each of the layers has a plurality of secondapertures, and wherein the second apertures overlap on adjacent layersto form a plurality of substantially axial fluid passages.
 10. Theelectric machine stator of claim 9, wherein the first aperture overlapsone of the second apertures.
 11. The electric machine stator of claim 9,wherein the stator comprises a flow director configured to direct fluidflow into, or receive fluid flow from, a subset of the plurality ofsubstantially axial fluid passages.
 12. The electric machine stator ofclaim 11, wherein the flow director is a layer at one end of the statorcore.
 13. The electric machine stator of claim 1, wherein the fluidchannel includes: a first axial segment through the back iron portion ofa first subset of the plurality of layers; a first azimuthal segment inthe back iron portion of the first layer; a first radial segment beingthe first portion of the fluid channel; a second axial segment extendingthrough a respective tooth of each of a second subset of the pluralityof layers; a second radial segment within a tooth of a second layer ofthe plurality of layers; a second azimuthal segment in the back ironportion of the second layer; and a third axial segment through the backiron portion of a third subset of the plurality of layers.
 14. Theelectric machine stator of claim 1, wherein the plurality of layers is aplurality of laminations.
 15. The electric machine stator of claim 1,wherein the plurality of layers is a plurality of turns of an edge-woundstrip.
 16. The electric machine stator of claim 1, wherein the pluralityof layers is a plurality of turns of a face-wound strip.
 17. Theelectric machine stator of claim 16, wherein the teeth of the pluralityof teeth are narrower at a first end of the strip than at a second endof the strip, and wherein a width of a slot between adjacent teeth atthe first end of the strip is the same as a width of a slot betweenadjacent teeth at the second end of the strip.
 18. The electric machinestator of claim 1, wherein each tooth of the plurality of teeth of eachof the plurality of layers extends radially inward from the back ironportion.
 19. The electric machine stator of claim 1, wherein each toothof the plurality of teeth of each of the plurality of layers extendsradially outward from the back iron portion.
 20. The electric machinestator of claim 1, further comprising: a stator winding; and anelectrically insulating resin having a thermal conductivity greater thanabout 0.4 W/m/° C., wherein the resin fills, with a void fraction lessthan about 10%, a space between the stator core and the stator winding.21. The electric machine stator of claim 1, wherein each of theplurality of layers has an aperture, of a plurality of apertures, in arespective tooth, wherein the plurality of apertures includes the firstaperture, and wherein the plurality of apertures overlap to form asecond portion of a fluid channel, the second portion of the fluidchannel comprising the first portion of the fluid channel, and thesecond portion of the fluid channel being substantially axial.
 22. Theelectric machine stator of claim 1, wherein all of the layers of theplurality of layers are identical, and each layer of the plurality oflayers is clocked by one tooth pitch relative to an adjacent layer. 23.An electric machine comprising: a rotor having an axis of rotation; anda stator having an axis, the axis of the stator being the axis ofrotation of the rotor, the stator being separated from the rotor by andair gap and having: a stator core having a plurality of layers, each ofthe layers having a back iron portion and a plurality of teeth, a toothof the plurality of teeth of a first layer of the plurality of layershaving a first aperture forming a first portion of a fluid channel,wherein the fluid channel includes: a first segment, the first segmentbeing substantially parallel to the first layer, and the first segmentcomprising the first portion of the fluid channel; a second segment, thesecond segment being substantially parallel to a second layer of theplurality of layers; and a third segment, the third segment extendingthrough a tooth of a third layer of the plurality of layers, wherein:the third layer is between the first layer and the second layer, thethird segment connects the second segment and the first segment, afourth layer of the plurality of layers is at one end of the stator coreand has no aperture, in any tip of any tooth, that is in fluidcommunication with the fluid channel, and the fluid channel is not influid communication with the air gap.
 24. An electric machinecomprising: a rotor; a stator having a stator core having a plurality oflayers, each of the layers having a back iron portion and a plurality ofteeth; channel means for channeling a fluid through the teeth of thestator core; and pumping means for supplying the fluid to the channelmeans, a tooth of the plurality of teeth of a first layer of theplurality of layers having a first aperture forming a first portion of afluid channel, wherein the fluid channel includes: a first segment, thefirst segment being substantially parallel to the first layer, and thefirst segment comprising the first portion of the fluid channel; asecond segment, the second segment being substantially parallel to asecond layer of the plurality of layers; and a third segment, the thirdsegment extending through a tooth of a third layer of the plurality oflayers, wherein: the third layer is between the first layer and thesecond layer, the third segment connects the second segment and thefirst segment, a fourth layer of the plurality of layers is at one endof the stator core and has no aperture, in any tip of any tooth, that isin fluid communication with the fluid channel, the stator is configuredto interact with the rotor through an air gap, and the fluid channel isnot in fluid communication with the air gap.
 25. The electric machinestator of claim 1, further comprising: a first substantially axial fluidpassage extending at least one half the length of the stator, and beingobstructed at a first end of the stator, a second substantially axialfluid passage extending at least one half the length of the stator, andbeing restricted at a second end of the stator, opposite the first end,the fluid channel forming a connection between the first substantiallyaxial fluid passage and the second substantially axial fluid passage.26. The electric machine stator of claim 1, wherein the layers abutagainst each other.